The fabrication of semiconductor electro-optical or photonic devices requires many processing steps to transform an epitaxially grown wafer into an effective electro-optical device. Most of this processing occurs in wafer form and involves forming electrical contacts on both the epitaxial and substrate sides of the wafer. The wafer is cleaved or separated into longitudinal strips, or bars, which extend substantially the full width of the wafer. Each bar is mechanically subsequently cleaved into individual electro-optical devices. Most commonly, the epitaxial side is the p-doped side and the substrate side is the n-doped side. However, the epitaxial side may be the n-doped side when the substrate side is the p-doped side. Electro-optical devices include lasers and light emitting diodes (LEDs).
The current process of semiconductor mirror formation, which involves separating a wafer into bar form (where one of the cleaved surfaces becomes the electro-optical device's mirror or other edge emitting surface), requires scribing the epitaxial side of the wafer to create a point or line of stress concentration to direct the cleavage location. In a conventional method of electro-optical device fabrication, this step produces "hackle," or microscopic steps in the bottom half of the mirror surface which typically causes little or no problem in device performance or yield. Sometimes, however, the hackle may run parallel to the top surface of the device and pass though the active region (top half of the mirror surface), and thereby cause failure and yield reduction. This excessive hackle may also result in parallel propagation to the top surface from the edge of the cleavage point and extending from the substrate side to the epitaxial side of the wafer. The currently employed process also requires slow and costly manual manipulation of the semiconductor bars prior to, and in the course of, separating the bars into individual devices.
The current method of separating semiconductor electro-optical devices from bar form to individual devices requires the incision of short scribe marks at the sides of each device. Rather than scribing the entire width of what is to become the new cleave, a short side scribe is centered on the bar without extending to either facet. Ordinarily, the cleave will follow the entire width of the bar or wafer without the need to scribe the entire width. Side scribing, or scribing only a portion of the width of the cleave path, minimizes some of the deleterious effects of scribing on the integrity of the wafer or bar material.
Scribe marks, however, provide no electrical isolation of individual adjacent devices from one another while the electro-optical devices are still in bar form. Electrical isolation between electro-optical devices is desired since it permits more efficient testing. Moreover, the method of side separation of bars into electro-optical devices based on the creation of scribe marks often results in non-straight edges of the electro-optical device, which can reduce yield and degrade overall chip appearance. Further, the scribing of the bar in the conventional fabrication process often weakens the bar, which results in breakage and yield loss, and may also create unwanted oxide dust that could impair visual inspection of cleaved bars and devices.
It is an object of this invention to provide an improved method of fabricating semiconductor electro-optical devices in which a wafer may be cleaved into bars having mirror facets substantially free of hackle and thus of higher quality and yield.
It is a further object of the invention to provide an improved method of fabricating semiconductor electro-optical devices in which marking the electro-optical device bars for further cleaving into individual devices may be performed more rapidly, and with a higher yield.
It is another object of the invention to provide a method of the type described which achieves electrical isolation of each electro-optical device, better epitaxial side appearance, elimination of oxide layer dust, and a stronger electro-optical device bar.
It is yet a further object of the invention to provide an improved method of fabricating semiconductor electro-optical devices in which cleaving the bars into individual devices may be performed by machine resulting in a faster processing time, higher yield, and better overall epitaxial and substrate side quality.